Better Software - June 2008 - (Page 14)

Code Craft A “D” in Programming, Part 2 by Chuck Allison As I write this, my last official pitch for the D programming language, I notice that D is at position twelve and climbing in Tiobe’s ranking of the twenty most popular languages for February 2008 [1]. Earlier in March, the first book on D, Learn to Tango with D, hit the shelves [2]. In its preface, D’s designer, Walter Bright, said: “Amazingly, there is no language that enables precise control over execution while offering modern and proven constructs that improve productivity and reduce bugs … Often programming teams will resort to a hybrid approach, where they will mix Python and C++, trying to get the productivity of Python and the performance of C++. The frequency of this approach indicates that there is a large unmet need in the programming language department. D intends to fill that need.” Like C++, D supports down-to-the-metal programming when you need it, and it compiles to fast-and-lean, native executables. It also supports generic programming and templates in all their glory. The standard C library is available directly from D code. Like Python, D supports modules and packages, garbage collection, functions that behave as first-class entities, a clean (though C-like) syntax, and flexible, built-in data structures. Like Java, D has inner classes. Like C#, D gives you delegates, but in a more flexible way. D is a multi-paradigm language that may be just what you need most of the time. For many of you, I suppose the software engineering features of D would be of most interest, but in this article I’d like to bring to “closure” (pun intended) a running example from previous Code Craft articles as I explore some powerful features of the D language. // MyClass.java class MyClass { private int theData; public int getData() { return theData;} } Listing 2 Nested Functions and Closures Much of what we enjoy today about objects was accomplished in earlier days through other means. To hide data in C, for example, you just declared a file-scope variable to be static. The file was the container, the “object,” if you will, that held data values not directly accessible to users. See listing 1. /* file1.c */ static int theData = 7; // Private data int getData() { return theData; } Listing 1 This approach breaks down when you need multiple instances. File I/O is too cumbersome and expensive to use as a model for objects that contain hidden data. Object-oriented languages have direct support for in-memory object creation at runtime, of course, as in listing 2. How did we ever survive without classes? There were many ways, but I’d like to mention one that is still important: nested functions. Nested functions are not 14 BETTER SOFTWARE JUNE 2008 permitted in many modern languages but were de riguer in languages like Lisp, Algol, PL/I, Pascal, and Ada. C dropped nested functions for simplicity, but Python and D have brought them back because sometimes they are superior to using objects. To illustrate, let me return to an example I’ve used in recent Code Craft articles: function composition. In the January 2008 Code Craft, I presented the generic C++ function composer shown in listing 3. A Composer is an object that takes a list of single-valued functions (or entities callable as such) and calls them in turn in nested fashion so you end up with f1(f2(…fn(x)…)). The private data here is the sequence of functions, indicated by the pair of iterators, beg and end. Since objects aren’t the only way to hide data, let’s look at the nested-function solution in D shown in Listing 4. This rendition of compose is a function template, evidenced by the (T) following its name, and it accepts a dynamic array of functions, each of which takes a single T argument and returns a T value. It returns a delegate that is callable as a singlevalued function of type T. The nested function doit iterates through the list of functions, funs, in reverse order, applying each function and accumulating the result as it goes. Let’s examine this more closely. Notice that compose has only two statements: a definition of the nested function doit and a statement that returns a www.StickyMinds.com ISTOCKPHOTO http://www.StickyMinds.com

Table of Contents Feed for the Digital Edition of Better Software - June 2008

Better Software - June 2008 Contents Mark Your Calendar Contributors Technically Speaking eLightenment Code Craft Test Connection Management Chronicles Agile Model-Driven Development The Myth of Risk Management Stop the Insanity! Product Announcements 10 Things You Might Not Know About … The Last Word Ad Index

Better Software - June 2008

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